1. Field of the Invention
This invention relates to an inductor for an integrated circuit structure characterized by a high magnetic susceptibility cobalt/nickel core material, and a method of making the inductor.
2. Description of the Related Art
In the formation of integrated circuit structures, active devices such as MOS and bipolar transistors are formed either in the semiconductor substrate or on layers, such as epitaxial layers, formed over the substrate. Passive electronic devices such as resistors and capacitors are also formed either within the semiconductor substrate or on/in conductive materials formed over the surface of the semiconductor substrate.
While inductors also comprise devices utilized in the formation of electronic circuits, their formation in integrated circuit structures has been more difficult, due to the large area needed to form an inductor with a useful amount of inductance and a high enough Q-factor, due to the low inductance of the materials conventionally used to form the inductor.
However, the formation of inductors on silicon substrates has been explored. Yue et al., in an article entitled A Physical Model for Planar Spiral Inductors on Silicon, published in IEDM-96, at pages 155-158, discusses the formation of an inductor on a silicon substrate, but does not discuss the use of a magnetic core material with the inductor; while Mark G. Allen, in an article entitled Integrated Inductors for Low Cost Electronic, published in IEDM-95, at pages 137-141, describes various shapes and configurations for inductors formed on a multichip module (MCM) substrate using iron-nickel and air coils. Ling U.S. Pat. No. 5,576,680 discloses a process for forming an inductor on a semiconductor substrate wherein first parallel strips of conductive material are deposited over a dielectric layer, which may be a layer of non-conductive high magnetic susceptible material (HMSM). A second layer of a dielectric (which may also be a non-conductive high magnetic susceptible material) is then formed over the parallel strips. If this second layer of dielectric material is not a high magnetic susceptible material, a layer of high magnetic susceptible material is formed over the second dielectric material (to form the core of HMSM of the inductor) and a further dielectric layer is formed over the high magnetic susceptible material. In either instance, filled vias are formed through the dielectric layer(s) down to the ends of the first parallel strips of conductive material. A further layer of metal lines is then formed over the structure and in contact with the filled vias to connect the opposite ends of the first strips of conductive material to thereby form a spiral coil surrounding the HMSM core. The type of material which may constitute this layer of HMSM is never identified, however, by Ling.
Ashby et al. U.S. Pat. No. 5,635,892 discloses the formation of inductors on or within a substrate material such as a semiconductor material or a dielectric material using a high magnetic permeability material such as iron spaced from a spiral conductor by a dielectric layer. Special configurations of the high magnetic permeability core material are discussed for lowering the magnetic coupling and to reduce eddy current losses.
Sundaram et al. U.S. Pat. No. 5,372,967 describe the formation of an inductor or a transformer in a series of parallel spaced apart trenches formed in an insulator or a semiconductor substrate. The trenches are lined at the bottom and opposite ends with a conductor. The walls between the trenches may then be removed and the resulting large single trench may be filled with a dielectric core material. Alternatively, a high permeability magnetic material such as ferrite may be used as the core material, in which case the core material is insulated from the conductor by a dielectric liner. Metal interconnects are then used to couple together opposite ends of the respective conductors in the trenches to form a long spiral conductor above and below the core material.
Volz U.S. Pat. No. 4,649,755 discloses the formation of an inductor wherein narrow conductive strips comprising a first portion of a coil are formed over a substrate, a first dielectric layer is formed over the first portion of the coil, and a magnetic core in the form of a ring is formed over the first dielectric layer. A second dielectric layer is then formed over the magnetic core and a second set of conductive strips, connected to the first conductive strips, is formed over the second dielectric layer. Materials said to be suited for the magnetic core are ferromagnetic amorphous metals. Good ferromagnetic properties are said to be exhibited by amorphous metals which are alloys based on transition metals of the iron group. Especially suited are said to be amorphous metals which are Co-Fe-base alloys, preferably Co.sub.x Fe.sub.y B.sub.100-x-y where 70.+-.x.+-.80 and 4.+-.y.+-.10.
Thus, the formation of inductors on semiconductor substrates is not unknown. However, the use of iron as the magnetic material for the core of the inductor, or at least an iron-containing magnetic core such as ferrite or an iron-containing metal alloy, seems to be considered essential to the formation of an inductor on a semiconductor substrate with sufficient inductance and high Q factor. While the use of iron or iron-containing material as the high magnetic susceptible material for the core of an inductor is well-recognized in the electronics industry, the formation of integrated circuits structures requires the judicial selection of materials which will be compatible with other materials already in use in the fabrication of integrated circuit structures on semiconductor substrates. Unfortunately, iron is not one of the materials in widespread use in the formation of integrated circuit structures, at least in part due to the ease with which iron is oxidized in the presence of either air or moisture.
Quite surprisingly, we have discovered an inductor for an integrated circuit structure characterized by high inductance and a high Q-factor which can be constructed as a part of the integrated circuit structure without the need to utilize an unreasonable amount of the area of the semiconductor substrate, and which can be constructed with a high magnetic susceptibility metal core without the introduction of a foreign material into the integrated circuit structure.